Pressure sensitive adhesive tapes and labels prepared by using a pressure sensitive silicone adhesive have been used under severe conditions where organic adhesives such as acrylic adhesives and rubber adhesives would be denatured and deteriorated since the layer of the pressure sensitive silicone adhesive has excellent heat resistance, cold resistance, weatherability, electric insulation, and chemical resistance. In addition, pressure sensitive silicone adhesives exhibit good adhesion to various substrates, and therefore, they are used for adhesion onto a substrate such as polyolefin resin, silicone resin, fluororesin, or a water-bearing surface which could not be adhered by organic pressure sensitive adhesives such as acrylic resin or rubber adhesion.
For example, adhesive tapes prepared by using a pressure sensitive silicone adhesive are used in the production, processing, and assembly of electronic and electric components. When heat is applied in such production process, entire or partial protection, re-masking, or temporary fixing of the electronic and electric components is often required. In the case of a pressure sensitive adhesive tape used for such application or the tape used in the production, fixture, or bundling of electronic and electric components exposed to a high temperature condition, preferred is the use of pressure sensitive adhesive tapes prepared by using a pressure sensitive silicone adhesive which retain their heat resistance even after heating to a temperature in excess of 250° C. In such applications, the pressure sensitive silicone adhesive is used as a pressure sensitive silicone adhesive tape prepared by coating a plastic film substrate with such adhesive to a thickness of several dozen μm.
In the recent production of electronic and electric equipment such as mobile phone, a pressure sensitive adhesive tape is quite often used as a means for fixing the parts since such use enables production of a thinner equipment with reduced weight. In such equipment, a shock-absorbing or stress-relaxing buffer member in the form of a sheet having a thickness of 0.1 to several mm is often incorporated for the purpose of preventing damage of the equipment or the interior parts by falling, pressing, or vibration, and such member is often fixed by using a double sided pressure sensitive adhesive tape or sheet. Such case, however, requires additional step of adhering the buffer member by using a pressure sensitive adhesive tape or sheet or additional preliminary multi-step preparation of a laminate comprising a film substrate having the buffer member layer on one surface and a pressure sensitive adhesive layer on the other side, and the assembly and processing often became complicated. Accordingly, there has been a demand for a buffer member having an adhesiveness that would enable the adhesion of member itself to various parts or casing.
Adhesion of members each having a size of several cm or more using a conventional pressure sensitive silicone adhesive was also associated with the problem of difference in the degree of expansion between the members due to the different expansion coefficient. More specifically, expansion and shrinkage of the members by repetitive temperature change resulted in the strain or deformation of the members as well as interfacial peeling between the member and the pressure sensitive adhesive. Conventional pressure sensitive silicone adhesives had been insufficient for use as a pressure sensitive adhesive member having the buffering property capable of absorbing such change.
In addition, conventional pressure sensitive silicone adhesives are mostly solvent type adhesives which suffer from foaming during the thermal curing when coated and cured or molded to a thickness of 0.1 mm or more. Accordingly, use of a solventless pressure sensitive silicone adhesive is preferable for such application.
Another type of materials used for the shock-absorbing and stress-relaxing member are soft gel materials comprising resins such as urethane resin and acrylic resin. However, gel materials generally suffer from insufficient heat resistance, and yellowing occurred even at a temperature as low as 100° C. Accordingly, these materials could not be used for an optical material. These materials also suffered from insufficient flexibility at low temperature as well as insufficient adhesiveness at high temperature. As described above, there has so far been no material fulfilling the requirement as described above.
Exemplary heat resistant gel materials include silicone gels, and silicone gels exhibiting the pressure sensitive adhesion are disclosed, for example, in JP-A 2007-126576. However, these silicone gels are insufficient in the adhesion to the substrate, and they may be peeled when an exterior force is applied. The silicone gels are also insufficient in the coherence, and the gel may undergo cohesive failure in the reworking such re-attaching of the gel, and the gel often remained on the substrate.
Conventional pressure sensitive silicone adhesives can be produced in a sheet-form product with sufficient pressure sensitive adhesiveness by coating on a plastic substrate or the like. Such products, however, were insufficient in the flexibility, and this prevented their use for a buffer member. In addition, when such product was adhered to an article with surface irregularities, the surface of the pressure sensitive adhesive layer could not fully fit the surface irregularities, and the remaining air gaps often prevented the pressure sensitive adhesive layer from fully adhering to the article surface.
A silicone composition for an addition curable pressure sensitive adhesive containing a mixture of a polyorganosiloxane having an alkenyl group and a polyorganosiloxane having no alkenyl group, a polyorganosiloxane having R3SiO0.5 unit and SiO2 unit, and a polyorganosiloxane containing SiH group is disclosed in JP-A 2008-24777. While this composition is described to exhibit strong adhesion to silicone rubbers, this composition which is a solvent-type composition was inferior in the flexibility and had little buffering function.
In addition, an addition curable pressure sensitive silicone adhesive composition containing a component prepared by partial condensation of (A) a raw rubber-like diorganopolysiloxane having hydroxyl group bonded to the silicon atom but no alkenyl group, (B) a raw rubber-like diorganopolysiloxane having an alkenyl group, (C) a polyorganosiloxane comprising R3SiO1/2 unit and SiO2 unit and containing at least 1.8% by weight of hydroxyl group bonded to the silicon atom, and (D) a polyorganosiloxane comprising R3SiO1/2 unit and SiO2 unit containing less than 1.8% by weight of hydroxyl group bonded to the silicon atom; and an organopolysiloxane having hydrogen atom bonded to the silicon atom is disclosed in JP-A 2004-231900. While this composition is said to exhibit a high pressure sensitive adhesion and a low tack, this composition which is a solvent-type composition was also inferior in the flexibility and had little buffering function. Furthermore, this composition had a drawback that, when one side of the sheet was several cm or more and the area was several dozen cm2 or more and the adhesion was beyond certain level, the adhered sheet could not be peeled off for reworking that is sometimes required to correct the erroneous positioning.
Another solventless pressure sensitive silicone adhesive disclosed is disclosed in JP-A 2008-274251. This composition comprises an alkenyl group-containing polydiorganosiloxane, a polyorganohydrosiloxane having 3 SiH groups, a polydiorganosiloxane having a low degree of polymerization having an alkenyl group at opposite ends, a polydiorganosiloxane having a low degree of polymerization having a SiH group at opposite ends, a polyorganosiloxane comprising R3SiO1/2 unit and SiO2 unit, and a platinum based catalyst. This composition exhibited high re-peelability and adequate pressure sensitive adhesion, and since the composition is solventless, it could be coated into a thick film. However, the resulting film was insufficient in the flexibility and buffering function.
The solventless pressure sensitive silicone adhesive disclosed in JP-A 2006-160923 was also insufficient in the flexibility and buffering function.
Another solventless or low solvent-type pressure sensitive silicone adhesive is disclosed in JP-A 2004-506778. This low solvent-type pressure sensitive silicone adhesive contains an alkenyl-terminated polydiorganosiloxane, a silanol-terminated polydiorganosiloxane, a resin copolymer containing R3SiO1/2 unit and SiO2 unit, an organohydrogen polysiloxane, a hydrosilylation catalyst, and an organic peroxide or an organic azo compound. This composition exhibits high pressure sensitive adhesion and tack, and the substrate may become deformed or damaged in the reworking or reattachment. The composition also suffered from the problem of foaming when the cured product was used at a high temperature or in the vacuum because of the decomposition residue remaining in the cured product due to the organic peroxide or organic azo compound used for the curing.
Another solventless pressure sensitive silicone adhesive is disclosed in JP-A 2006-520838. This adhesive is a solventless pressure sensitive silicone adhesive which contains an organic siloxane polymer having an aliphatic unsaturated group, a resin having R3SiO1/2 unit and SiO2 unit, a reactive diluter agent, an SiH-containing crosslinking agent, a hydrosilylation catalyst, and an inhibitor. This composition is also insufficient in the flexibility and buffering function. This adhesive also suffered from the problem caused by the use of a reactive diluent such as an alkene having a terminal double bond. More specifically, bubbles often remained in the cured product due to the foaming during the curing of the thick film, and when the diluent remained unreacted in the adhesive, the cured product suffered from foaming in the use of the cured thick film at a high temperature or in the vacuum.